The present invention relates to a process for the preparation of superazeotropic hydrochloric acid from metal chloride solutions as well as to a method for processing ores and/or metals utilizing superazeotropic hydrochloric acid prepared according to the invention.
The term xe2x80x9csuperazeotropicxe2x80x9d in this context refers to a mixture of water and hydrogen chloride, the content of hydrogen chloride of which is higher than at the azeotropic point of the system water/hydrogen chloride under normal pressure. The azeotropic point of the system under normal pressure is at approximately 20% by weight HCl.
Regeneration of hydrochloric acid from spent solutions containing metal chloride resulting from ore and/or metal processing, e.g. solutions from leaching of ores or spent pickling acids, is known in many ways from the prior art.
In particular, the regeneration of hydrochloric acid by thermal decomposition of the solutions containing metal chloride is known, e.g. by pyrohydrolytic treatment wherein the hydrochloric acid is attained in a gaseous state in the exhaust vapors and wherein the corresponding metal oxide may be recovered as a further product.
The hydrochloric gases so recovered are mostly absorbed in water, thereby obtaining hydrochloric acid. A characteristic of this prior art process is the fact that the hydrochloric acid recovered is mostly sub-azeotropic, i.e. that the proportion of hydrogen chloride is below the azeotropic point of the system water/hydrogen chloride under normal pressure.
The reason for this is that during the adiabatic adsorption of the hydrochloric acid from hydrochloric gases the azeotropic point cannot be exceeded due to the chloride content of the starting solution.
Typical proportions of HCl in the hydrochloric acid obtained in processes according to the state of the art are e.g. in the region of 18% to 20% by weight, if the chloride content of the starting solution is about 200 g/l.
Only in exceptional cases where the chloride concentration in the solution to be regenerated is so high that a superazeotropic mixture is present already after the thermal decomposition in the hydrochloric gases, may a hydrochloric acid, the content of HCl of which is above the azeotropic point, be recovered by condensation alone.
In the event of a chloride content in the starting solution of 300 g/l a hydrochloric acid having approx. 26% to 27% by weight may be recovered by condensation alone from the roasting gas of a pyrohydrolytic treatment.
An increased need has, however, come about recently to prepare superazeotropic hydrochloric acid from solutions containing metal chloride which, as in the first case mentioned above are so composed that by a mere absorption of the hydrochloric gas only a sub-azeotropic mixture is obtained.
A need exists, however, even for the second case mentioned above where superazeotropic mixtures are already obtained per se, to obtain a more highly concentrated hydrochloric acid.
Concentrated hydrochloric acid having a HCl-content of, for example, 30% by weight or more may be employed for a greater variety of purposes than a more diluted hydrochloric acid. A concentration of hydrochloric acid having a HCl content of 26% to 27% by weight is, however, not possible by mere evaporation of excess water.
Ways and means are described in the literature as to how, from a diluted hydrochloric acid, the HCl-concentration can be moved above the azeotropic point of the system HCl/H2O by shifting the H2O partial pressure. In this context one may avail oneself e.g. of an entrainer which binds water and thereby shifts the ratio HCl:H2O. It is further known to obtain superazeotropic hydrochloric acid by employing different pressures.
A process to this effect is disclosed in Ullmann""s Enzyklopxc3xa4die der Technischen Chemie (Encyclopaedia of Technical Chemistry), 3rd edition, 1964, volume XV, page 79. In this case a highly concentrated hydrochloric gas is discharged in a first tower at a pressure above atmospheric. The bottom product (diluted hydrochloric acid) is treated in a second tower at reduced pressure in which process water is evaporated off and diluted hydrochloric acid is obtained as the bottom product with the azeotropic composition typical of the reduced pressure in the second tower. This diluted hydrochloric acid can now be reintroduced to the first tower.
In order to move from the sub-azeotropic hydrochloric acid obtained by means of the known methods mentioned above or from the relatively low concentrations of superazeotropic hydrochloric acid towards concentrated superazeotropic hydrochloric acid, the prior art has up to date proposed to upgrade the hydrochloric acid obtained by the condensation or absorption of the entire hydrochloric gases in a separate installation.
However, this separate upgrading proves to be cost-intensive so that, instead of upgrading the regenerated hydrochloric acid one opts frequently for discarding the acid and for purchasing fresh hydrochloric acid for the required purposes. In many instances an 18% regenerated hydrochloric acid is thus not used any further but is neutralized which, in turn, may cause environmental problems due to the metals possibly still contained in traces in the hydrochloric acid, such as e.g. chromium or nickel.
U.S. Pat. No. 4,216,196 discloses a process in which from the total volume of the hydrochloric acid condensed from the hydrochloric gases a partial stream is upgraded to highly-concentrated HCl-gas by means of the above described entrainer method and in which this gas is reunited with the main stream of the condensate. A solution of that metal chloride which is also fed to the thermal decomposition is employed as an entrainer.
It is the object of the present invention to obtain superazeotropic hydrochloric acid from the regeneration of solutions containing metal chloride by means of a cost effective process.
This object is attained according to the invention in that the metal chloride solutions are thermally decomposed to form a hydrochloric gas, in that the hydrochloric gas is processed into a superazeotropic gas mixture of hydrogen chloride and water in a manner known per se, the superazeotropic gas mixture is condensed to yield a superazeotropic hydrochloric acid.
For the purposes of the present invention the term xe2x80x9cprocessing into a superazeotropic gas mixturexe2x80x9d in this context either means the upgrading of a hydrogen chloride water mixture having a sub-azeotropic composition into a superazeotropic mixture or the further upgrading of an already superazeotropic mixture. By xe2x80x9csuperazeotropic gas mixturexe2x80x9d it is understood that the hydrogen chloride concentration in the gas mixture is so high that hydrochloric acid having the desired superazeotropic composition comes about after a condensation of the mixture.
The method according to the invention differs from the prior art methods in that the hydrochloric gases obtained by the thermal decomposition of the metal chlorides are upgraded to a superazeotropic gas mixture prior to their condensation so that, contrary to the prior art, it is not necessary to condense or absorb in water the entire hydrochloric gas and the hydrochloric acid obtained does not subsequently have to be treated or upgraded to a superazeotropic hydrochloric acid.
This offers the advantage that expensive upgrading plants may be kept smaller than in the case of processes according to the prior art. It is also found that the energy expenditure can be kept lower with the process according to the invention than in the case of prior art processes.
By performing the process according to the invention, the problems of the prior art are avoided in an astonishingly simple manner, enabling the person skilled in the art to prepare in a favorable manner even higher concentrations of superazeotropic hydrochloric acid from solutions containing metal chloride.
The process according to the invention is preferably so designed that the metal chloride solution is thermally decomposed in a spray-roasting-reactor or in a fluidized bed reactor. Processes of this type, e.g. the pyrohydrolytic decomposition of the solution containing metal chloride into the corresponding metal oxide and into hydrochloric gases are known per se from numerous prior art documents.
In an advantageous manner a partial stream of the hydrochloric gas is condensed or absorbed, processed in a manner known per se into a superazeotropic gas mixture and the superazeotropic gas mixture obtained is recombined with the main stream, the volume of the partial stream and/or the concentration of the resulting superazeotropic gas mixture being so selected that following the recombination of the resulting superazeotropic gas mixture with the main stream the resulting gas mixture possesses the desired superazeotropic composition.
A defined volume of hydrochloric gas is therefore separated from the main stream, e.g. by partial condensation, the said partial stream being processed into a superazeotropic gas mixture and water vapor by means of processes known from the prior art. The resulting superazeotropic gas mixture is united with the main stream of the hydrochloric gas. By the appropriate selection of the volume of the partial stream, the concentration of hydrogen chloride, respectively, to which the partial stream is upgraded, the desired concentration of the resulting gas mixture obtained by the recombination of the partial stream with the main stream may be set.
This, in particular, offers the advantage that only the water content withdrawn from the main stream, needs to be evaporated a second time for which reason, according to this preferred embodiment, the energy expenditure may be kept even more substantially lower.
The hydrochloric gas or the phial stream respectively of the hydrochloric gas is dewatered preferably by employing different pressures e.g. as described further above.
Particularly when using the process with different pressures (xe2x80x9cpressure methodxe2x80x9d) the preferred embodiment, wherein only a partial stream of the hydrochloric gases is processed, is particularly suitable.
That is to say, the pressure method proves to be more advantageous with regard to energy when carried out on a partial stream than the alternatively possible entrainer method. Furthermore, the pressure method offers the advantage that no foreign substances acting as entrainers are introduced into the process. It is moreover not necessary, contrary to the case of the entrainer method, to free the auxiliary substance (the entrainer) from HCl impurities. A further advantage of the application of the pressure method, as opposed to the entrainer method resides in that it is not necessary to purchase additional substances as entrainers. If, on the other hand, as e.g. in U.S. Pat. No. 4,216,916 , that metal chloride solution is used as an entrainer which is also thermally decomposed, the efficiency of the process depends considerably on the type of the available metal chloride solutions (e.g. chlorides of the metals Al, Fe, Cr, Ni, Mn, Co). This drawback also falls away if the pressure method is applied.
If the known entrainer method is utilized for upgrading hydrochloric gas, or the partial stream, suitable entrainers are e.g. concentrated sulphuric acid or a concentrated metal chloride solution. The metal chloride solution may be branched off that metal chloride solution which is thermally decomposed.
For the separation of the partial stream the hydrochloric gas may be absorbed in water in known manner.
Preferably, the hydrochloric acid obtained according to the invention comprises a hydrogen chloride content of more than 27% by weight, preferably 30% to 36% by weight, particularly preferably 32% by weight.
The object of the present invention is further attained by a process for ore and/or metal processing, e.g. for pickling of steels, in particular special steels, and/or for ore leaching by means of hydrochloric solutions, which is characterized in that the metal chloride solutions are thermally decomposed with the formation of a hydrochloric gas, that the hydrochloric gas is condensed of, respectively, absorbed in water, that a partial stream of the hydrochloric gas or, as the case may be , the hydrochloric acid obtained from the hydrochloric gas by condensation or absorption is split into hydrochloric acid gas and water or diluted hydrochloric acid respectively, by applying different pressures and the resulting hydrochloric acid gas is recombined with the main stream of the gas or, respectively, with the hydrochloric acid obtained from the hydrochloric gas, the volume of the partial volume being so selected that the resulting mixture shows the desired superazeotropic composition when recombining the hydrochloric acid gas with the main stream.
As already mentioned above, the procedure of upgrading a partial stream taken either from the hydrochloric gases or, in the event of the hydrochloric gases having been condensed or absorbed in water already, from the hydrochloric acid obtained thereby, permits an energetically particularly advantageous possibility for the preparation of superazeotropic hydrochloric acid by way of the described pressure method.
In a particularly efficient manner this process is so designed that as a partial stream, a partial stream from the hydrochloric acid obtained from the hydrochloric gases by absorption or condensation, is used for which purpose the treatment of the partial stream of the resulting hydrochloric acid is performed directly after the absorption or condensation of the hydrochloric gases.
For the purposes of the present invention the term xe2x80x9cdirectlyxe2x80x9d means in this context that the processing of the hydrochloric acid obtained from the hydrochloric gases is performed in a plant which is not separate from the regeneration plant. The processing of the hydrochloric acid according to this advantageous embodiment rather forms part of the process of thermal decomposition of the metal chloride solution and the condensation or, respectively, absorption of the hydrochloric gases so that e.g. energy flows or water/water vapor from the stages of this process may be summoned also for the processing of the partial stream.
A particularly preferred embodiment of the process according to the invention resides in employing the hydrochloric acid obtained according to the invention as a starting solution in one of those processes where solutions come about containing metal chloride from which, in turn the hydrochloric acid is regenerated. A cycle can thus be closed and the demand for fresh hydrochloric acid substantially lowered.
For the operator of an ore and/or metal processing plant, such as e.g. a pickling plant, the attractive possibility is offered to not only regeneratexe2x80x94as hitherto knownxe2x80x94hydrochloric acid from spent pickling liquors but to also obtain a more highly concentrated product by processing according to the invention the regenerated hydrochloric acid, which product may be reused either at the onset of the pickling process, but also for other purposes in the context of ore processing, e.g. leaching.
The steps of
ore and/or metal processing, e.g. a pickling installation and/or a leaching installation,
the recovery of hydrochloric gases from spent liquors of the ore and/or metal processing,
the preparation of superazeotropic hydrochloric acid from the obtained hydrochloric gases according to the process of the invention and
the condensation or adsorption of the resulting hydrochloric gas mixtures
may thus be provided in a single plant.
Such a plant enables the ore and/or metal processor to cover the needs of the own enterprise for concentrated hydrochloric acid at least in part, but also to open up a further source of income by means of the superazeotropic hydrochloric acid obtained according to the invention and usable for a multitude of purposes.
The invention is further elucidated in what follows by way of the figures and working examples.